An experimental study of basalt–seawater–CO₂ interaction at 130 °C

Over millions of years, the interaction of marine basalt with percolating seawater in low-temperature ocean floor hydrothermal systems leads to the formation of calcite and aragonite. The presence of these minerals in marine basalts provides evidence for substantial CO₂ fixation in these rocks. Here, we report on laboratory experiments to study this process under enhanced CO₂ partial pressures (pCO₂) at 130 °C. Mid-ocean-ridge-basalt (MORB) glass was reacted with North Atlantic Seawater charged with CO₂ in batch experiments lasting up to 7 months. For experiments initiated with seawater charged with ~2.5 bar pCO₂, calcite and aragonite are the first carbonate minerals to form, later followed by only aragonite (±siderite and ankerite). For experiments initiated with seawater charged with ~16 bar pCO₂, magnesite was the only carbonate mineral observed to form. In total, approximately 20% of the initial CO₂ in the reactors was mineralized within five months. This carbonation rate is similar to corresponding rates observed in freshwater-basalt-CO₂ interaction experiments and during field experiments of the carbonation of basalts in response to CO₂-charged freshwater injections in SW-Iceland. Our experiments thus suggest that CO₂-charged seawater injected into submarine basalts will lead to rapid CO₂ mineralization. Notably, at pCO₂ of tens of bars, magnesite will form, limiting the formation of Mg-rich clays, which might otherwise compete for the Mg cation and pore-space in the submarine basaltic crust. This suggests that the injection of CO₂-charged seawater into subsurface basalts can be an efficient and effective approach to the long-term safe mineral storage of anthropogenic carbon.